We are as far removed in time from the inauguration of John F. Kennedy as that day was from the presidency of William Howard Taft. To those of us who were young in 1961, Taft seemed like ancient history.

Yet as the nation marks the 50th anniversary of JFK's summons to a new generation, his presence and his presidency still speak, at times powerfully, to succeeding generations. In part, this is because the medium constantly gives new life to the message and the man — in film, on television, nowhere more vividly than in the sights and tones of his own speeches and press conferences and almost singularly in the inaugural address.

This resonance is a possibility of modern presidency, but not a certainty. Many of Kennedy’s successors, fairly or unfairly, already exist far more in scholarly precincts than in the popular consciousness.

Nor is his endurance, as glib critics and the resentful Right suggest, simply an artifact of "image." That notion reaches back to a myth about the first Kennedy/Nixon debate, which frustrated partisans invoked to console themselves in defeat: The majority of Americans who saw the debate thought Kennedy won, but they must have been distracted by his charm and his opponent’s on-camera perspiration because the minority who only listened on radio gave the victory to Nixon. It’s an easy analysis — and a rationalization repeated to this day. It also ignores a decisive reality. Who would choose to listen on radio to the first presidential debate in history rather than watch it on television? Almost entirely those who didn’t have television service: They were disproportionately remote, rural, Protestant — and inclined to favor Nixon in the first place.

Both substance and spirit are essential to greatness in a leader.

Critics also argue that the Kennedy presidency has an unfair claim based on its words and not its works. It is true that the time was brief. Kennedy said in his inaugural speech that "all this will not be finished in the…first thousand days." His life and his presidency were when a few moments in Dallas scarred and altered our history.

But the eloquence of his entry into office, and the tragedy of his leaving, should not subtract from the deeds that are a measure of presidential greatness.

In the Cuban Missile Crisis, the most perilous confrontation of the Cold War, his combination of strength and restraint stayed the onset of what he called "mankind’s final war." In those 13 days, as he defied the advice of many in the military and others involved in the secret White House deliberations, he arguably saved the world. Afterwards, in the last summer of his life, without checking the initiative with the bureaucracy, he proposed negotiations with the Soviets and then signed and secured the ratification of the Nuclear Test Ban Treaty. The moment of maximum danger was succeeded by the first true arms-control agreement of the post-war period — the beginning of a process which continues to this day.

Elsewhere Kennedy challenged the counsels of caution, setting America on course to land men on the moon and to explore the new frontier of space. Following a decade of repeated recession, he discarded the conventional wisdom and got the country moving again by demanding that an activist government stimulate and sustain the prosperity of the nation. The key legislation didn’t pass until after his death, but he was the first president, and perhaps the only one still, who dared to be explicitly Keynesian.

On civil rights, he was both cautious and courageous, waiting for the right time but each time making the right decision. He ordered the armed forces to integrate the University of Mississippi and then the University of Alabama. Warned that his party could lose the South for a generation and more — and it has — he proposed the Civil Rights Bill in 1963 and became the first president to declare that equality was "above all, a moral issue." Like Lincoln, he was a politician and he maneuvered his way forward. But black Americans knew where he stood. That’s why his picture was on the walls of their homes. And that’s why Lyndon Johnson, in his first speech to Congress, insisted that "no memorial…could more eloquently honor President Kennedy’s memory than the earliest possible passage of the Civil Rights Bill."

Kennedy had his defeats — on Medicare in 1962 — and his mistakes. But he also had a capacity, rare in politicians and presidents, to publicly admit when he was wrong. He took responsibility — which certainly wasn’t entirely his — for the failure of the CIA’s Bay of Pigs invasion of Cuba. He was ready to reverse established policy — as he did when he negotiated with the Communists for the neutralization of Laos rather than sending U.S. forces in. And the evidence now is that had he lived, we could have been spared a decade of death in Vietnam and Southeast Asia.

I cite all this because my final point should be seen not as a substitute for presidential achievement, but as central to it. Words do have power — and they relate to a fundamental test of greatness in presidents: whether they enlarge and enrich America’s conception of itself. This is as much or more a mark of greatness as bills proposed or passed, or crises met.

And in their greatness, Kennedy’s words — at the inauguration and then for the next two years, 10 months, and 22 days — not only imprinted his memory on ours, but inspired us to see our country in a new light and to change it in ways that continue to this day. His words gave rise to deeds—not the mere acts of officials, but an era of courage and commitment from people across America and across the decades. In the 1960s, they fought in war and served in the Peace Corps. They marched for civil rights — before Kennedy sent the legislation to Congress. Indeed, as he told Dr. Martin Luther King Jr., that movement made the legislation possible — and inevitable.

The idealism and energy let loose widened America’s span: in demands to protect the environment and to secure the rights of women, the ethnically different, and the differently abled (and now gays and lesbians) — causes barely perceived if at all on that Kennedy inaugural day.

But from that moment and from what followed came a profound heightening of the aspirations of citizenship — that each of us, in our own way, could make a difference and a different world. This still reverberates today. And this, too, was Kennedy’s great and lasting contribution.

From Lincoln to the Roosevelts to Reagan, there are presidents who have touched America's soul and reframed its mission, laying down touchstones for the future. Alongside substance stands spirit, and both are essential to greatness in a leader. As Ronald Reagan said of John Kennedy, "When he died, a comet disappeared over the continent…many men are great, but few capture the imagination and the spirit of the times. The ones that do so are unforgettable."

That is why those of us who were young in 1960 will never forget. And that is why half a century on, and a century and more from now, people will not only remember, but see the comet and be moved to ask again what they can do.